The present invention relates to a valve gear for charge cycle valves of internal combustion engines having a camshaft whose cam acts on a valve stem of a charge cycle valve via a bucket tappet which is guided in a cylinder head and on which an inner tappet is supported via a first pressure spring, and having a switchable coupling element, which is arranged between the bucket tappet and the circumference of the inner tappet, which bears against the valve stem, and permits force transmission from the bucket tappet to the valve stem.
The valve gears of the known type are used, on one hand, to switch off individual valves or a plurality of valves or, on the other hand, to switch over from one stroke characteristic to another in the case of the same valve. With spark-ignition engines, valves of individual cylinders are switched off in order to avoid the pumping losses at the switched-off cylinders and cooperate with a higher mean pressure in the case of the remaining, fired cylinders, as a result of which the thermal efficiency is raised. There is a corresponding reduction in the consumption and exhaust gas in the part load range.
With directly injecting diesel engines, the charge cycle causes substantially smaller losses. If, however, some of a plurality of inlet valves per cylinder can be switched off, the intake turbulence in the combustion chamber can be influenced as a function of rotational speed, and thus the combustion can be improved.
The stroke characteristic of a valve can be changed in many regards by switching over the valve, specifically both with reference to the opening stroke and with reference to the valve timing. Thus, the valve timings for two speed ranges can be optimized with the aid of two different stroke characteristics, the result being advantages as regards consumption and exhaust gas.
Numerous valve gears for charge cycle valves are known which have a device for switching off or switching over a charge cycle valve, for example DE 42 13 147 A1, DE 43 33 927 A1, DE 44 05 189 C2 and DE 44 43 101 A1. Two systems can be distinguished in principle in this case.
A first system a shown in, DE 44 43 101 A1, has, inside a tappet, a hydraulic fluid chamber which is filled with or emptied of oil in order to switch the charge cycle valve on and off, respectively. Transmission of the actuating forces requires a high oil pressure which renders a simple seal difficult and requires expensive provision of hydraulic fluid. Both the seal and the provision of hydraulic fluid call for a great deal of installation space.
In a second system, as shown in DE 44 05 189 C2, the charge cycle valves are turned on or off by self-closed, switchable coupling elements, for example pins, bolts or plates which are arranged between a bucket tappet and the charge cycle valve. The arrangement and actuation of the self-closed coupling elements likewise call for a substantial installation space. Moreover, an impermissibly high face-to-face pressure, and wear associated therewith, can arise at the coupling point when the self-closure achieved upon engagement is not complete. Furthermore, troublesome coupling noises can occur.